Surface hardening is a process of hardening the surface of a metal object while allowing the metal underneath to remain relatively soft, and generally involves forming a thin layer of a harder metal at the surface of the metal object. This may be accomplished by dispersing strengthening particles such as metal carbides, metal nitrides, metal oxides or hard metals into a metallic matrix to form a so-called dispersion strengthened alloy which is harder than the base matrix and may exhibit improved wear resistance. In some cases these strengthening particles may be dispersed throughout a substrate or metal object to improve surface hardness and overall strength.
One example of a dispersion strengthened alloy is NS-163™, which is a cobalt-based alloy formed by heat treating the base alloy (Co-28Cr-21Fe-9Ni-1.25Ti-1Nb) under an atmosphere of nitrogen to form a dispersion of metal nitride particles throughout the resulting alloy. Although the resulting dispersion strengthened alloy exhibits excellent stress-rupture strength at temperatures up to 1204° C., it also presents certain disadvantages with respect to its formation and workability. For example, the heat treatment strengthening process is limited to relatively thin substrates, e.g. about 2.5 mm maximum. Also, after fabrication and heat treatment the alloy cannot be effectively processed by forming or welding because such heating/melting processes lead to a reduction or elimination of the original strengthening effect. It is thought that this degradation occurs when heat from the subsequent metalworking process reduces, changes or eliminates the nitride dispersion that formed during the original heat treatment process. As a result, the nitride dispersions may be disrupted in that portion of the previously strengthened alloy subject to the subsequent metal working—thus causing the effected portion of the alloy to be weaker than a remainder of the alloy.
FIGS. 1 and 2 illustrate this problem. FIG. 1 depicts a prior art welding process for edge-to-edge joining of two dispersion strengthened metal substrates, such as NS-163, containing a dispersion of strengthening particles. In a typical, non-limiting example two dispersion strengthened metal substrates 2a,b are juxtaposed such that their respective edges form a joint 6 (exemplified in FIG. 1 as a single-V-groove). A filler material 8 containing an alloy material 10 may then be deposited into the groove 6, and subsequently melted by traversing an energy beam 12 across the surface of the filler material 8 to form a melt pool (i.e., weld pool) 14 within the groove 6. Upon cooling and solidification, the melt pool 14 forms a weld joint 16 that fuses the two ends of the metal substrates 2a,b together.
As explained above, the process of FIG. 1 introduces points of weakness into the resulting welded structure by causing a disruption of the particle dispersions within the previously hardened substrate material. FIG. 2 illustrates a cross-sectional view of the welded structure of FIG. 1, in which the weld joint 16 contains an area 22 having reduced and/or segregated dispersions 4. The segregation of nitrides within the melt zone is likely due in some part to differences in density between nitride particles (e.g., 5.22 g/cm3 for TiN and 8.4 g/cm3 for NbN versus 7.95 g/cm3 for alloy NS-163). Such nitrides have higher melting temperature than the general alloy (e.g., 2930° C. for TiN and 2573° C. for NbN versus 1288 to 1400° C. for the NS-163 substrate). So, upon the solidification of the nitrides, some nitride particles may segregate at a top surface and some may sink toward a bottom surface during the balance of weld solidification. Furthermore, areas 20 within the heat affected zone of the original matrix of the dispersion strengthened substrates 2a,b are also devoid of nitride dispersions, or contain nitrides having altered shape and size. The reason for this is likely due to diffusion of nitrogen at elevated temperature causing dissolution and agglomeration of nitrides and/or changes to their shape. As a result of segregations and alterations of nitrides in the weld joint 16 and adjacent heat affected zones, a weakened portion 24 results within, and in the vicinity of, the weld joint 16.
So, in summary, welding of dispersion strengthened alloys such as NS-163 is problematic because the heat of welding (e.g., arc, laser, plasma, etc.) can lead to the mal-distribution of the strengthening particles by dissolving and segregating them and/or by altering their size and shape. These combined effects result in a weld zone 24 of inferior properties relative to the original dispersion strengthened alloy.